1. Field of the Invention
The present invention is directed to a coupler for coupling light to a fiber while minimizing feedback to the light source due to reflection along the transmission system.
2. Description of Related Art
As the use of non-physical contact connections between light sources and fibers increases, the need for effective isolation to prevent light reflected at the fiber interface from being returned to the light source increases. Feedback to the light source may result in spectral broadening, light source instability, and relative intensity noise, which affect the monochromaticity of the light source. As data rates go up, the systems become more sensitive to relative intensity noise and require low bit error rates. Conventional optical isolators using polarization effects to attenuate reflection are very expensive, making the non-physical contact impractical. The importance of avoiding feedback is further increased when trying to use cheaper light sources, such as vertical cavity surfaces emitting laser diodes and light emitting diodes.
One solution that avoids the use of an optical isolator is a mode scrambler that divides power from the light source into many modes. A configuration employing a mode scrambler includes a single mode pigtail that provides light from the light source to the mode scrambler that then delivers the light to a transmission cable via an air-gap connector. Since any reflected power will still be divided across the many modes, any reflected power in the mode that can efficiently be coupled into the pigtail is only a small fraction of the total reflected power, thereby reducing return losses. However, this solution involves aligning another fiber, physically contacting the fiber with the mode scrambler, and placing the light source against the fiber. This pigtailing is expensive. Thus, there still exists a need for true non-physical contact connection between a light source and a transmission system that does not require an isolator.
The present invention is therefore directed to a coupler between a light source and a transmission system that substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
These and other objects of the present invention may be realized by providing an apparatus for coupling light from a light source at an input plane into an optical fiber including an optical element which couples light from the light source to the optical fiber and which returns light traversing the optical system back towards the input plane such that the returning light does not substantially overlap with an output of the light source in the input plane.
The optical element may be a mode matching element, wherein light output from the optical element is distributed in a desired angular distribution that is substantially maintained along the fiber for more than a depth of focus of the optical element. The mode matching element may be a diffractive element. The mode matching element may be a refractive element. The optical element may have first and second surfaces, with the mode matching element being provided on the second surface, further from the light source. The apparatus may further include an angular distribution altering element on the first surface. The angular distribution altering element may provide a ring pattern on the second surface. The angular distribution altering element may be a diffractive surface having a radially symmetric lens function and a negative axicon function.
The optical fiber may be a multi-mode GRIN fiber and the optical element couples light into higher order modes of the multi-mode GRIN fiber to reduce differential mode delay. The optical element may be a diffractive surface having a radially symmetric lens function and a negative axicon function. The optical element may map a point from the input plane into more than one point on the optical fiber. The optical element may couple light to the optical fiber by directing light from the light source away from a center of the optical fiber.
The above and other objects of the present invention may be realized by providing a system for coupling light to an optical fiber including a light source at an input plane and an optical element which couples light from the light source to the optical fiber and which returns light traversing the optical system back towards the input plane such that the returning light does not substantially overlap with an output of the light source in the input plane.
The system may include a power monitor for the light source. The power monitor may monitor light output from a front of the light source. The system may include a deflecting element that deflects a portion of the light output from the front of the light source to the power monitor.
The above and other objects may further be realized by providing an apparatus including a reciprocal optical element that couples light from a light source in an input plane into a fiber and reduces coupling of light reflected back to the light source. The reciprocal optical element may be a single optical element. The single optical element may have at least two surfaces. The reciprocal optical element may be an axicon. The reciprocal optical element may be a mode matching element. The reciprocal optical element may map a point in the input plane to more than one point at the fiber. A change in phase of light passing back through the reciprocal optical element towards the light source may prevent light from substantially overlapping an output of the light source in the input plane.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the present invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility without undue experimentation.